Ultrafast excited state deactivation and energy transfer in guanine-cytosine DNA double helices

International audienceThe stability of DNA components with respect to UV radiation is considered to be a prerequisite for the development of the genetic code. But it is also known that UV light absorbed by DNA bases may damage the double helix and lead to carcinogenic mutations.1 The interplay between stability and photodamage depends on the way that the energy of a UV photon is distributed among the electronic excited states of the double helix before it is eliminated as heat. Ultrafast dissipation of the excitation energy is indeed a common property of all the monomeric DNA building blocks: the major part of the excited state population of nucleosides and nucleotides in aqueous solution lives for less than one picosecond.2,3 When applied to double helices, composed exclusively of adenine-thymine base pairs (A-T duplexes, both homopolymeric and alternating), femtosecond spectroscopy reveals a different picture: organization of the bases within duplexes causes an overall lengthening of the excited state lifetimes.4-9 This is due to the emergence of new excited states, shared between at least two bases. The existence of delocalized excited states allows ultrafast energy transfer to occur5,8-10 by-passing the prerequisites of Förster transfer which are not fulfilled in the case of DNA bases

Excited-state dynamics of dGMP measured by steady-state and femtosecond fluorescence spectroscopy.

International audienceThe room-temperature fluorescence of 2'-deoxyguanosine 5'-monophosphate (dGMP) in aqueous solution is studied by steady-state and time-resolved fluorescence spectroscopy. The steady-state fluorescence spectrum of dGMP shows one band centered at 334 nm but has an extraordinary long red tail, extending beyond 700 nm. Both the fluorescence quantum yield and the relative weight of the 334 nm peak increase with the excitation wavelength. The initial fluorescence anisotropy after excitation at 267 nm is lower than 0.2 for all emission wavelengths, indicating an ultrafast S(2) --> S(1) internal conversion. The fluorescence decays depend strongly on the emission wavelength, getting longer with the wavelength. A rise time of 100-150 fs was observed for wavelengths longer than 450 nm, in accordance with a gradual red shift of the time-resolved spectra. The results are discussed in terms of a relaxation occurring mainly on the lowest excited (1)pi pi*-state surface toward a conical intersection with the ground state, in line with recent theoretical predictions. Our results show that the excited-state population undergoes a substantial "spreading out" before reaching the CI, explaining the complex dynamics observed

Excited States and Energy Transfer in G-Quadruplexes

International audienceDNA nanostructures formed by association of four oligonucleotides d(TGGGGT) (TG4T quadruplexes) are studied by steady-state and time-resolved optical spectroscopy with femtosecond resolution using fluorescence upconversion. A comparison between single-stranded and four-stranded structures and the corresponding stoichiometric mixture of noninteracting nucleotides shows how horizontal and vertical organization affects the properties of the excited states. Emission from guanine excimers is observed only for single strands, where conformational motions favor their formation. Quadruplex fluorescence arises from a multitude of excited states generated via electronic coupling between guanines; the average fluorescence lifetime is longer and the fluorescence quantum yield higher compared to those of noninteracting nucleotides. The fluorescence anisotropy recorded on the subpicosecond time scale, where molecular motions are hindered, reveals that energy transfer takes place among the bases composing the nanostructure. These results are in line with the conclusions drawn from similar studies on model DNA duplexes

Single - and double energy swift and slow heavy ion irradiated optical waveguides in Er: Tungstene-Tellurite glass and BGO for telecom applications

The fabrication of broadband amplifiers in wavelength division multiplexing (WDM) around 1.55 m, as they exhibit large stimulated cross sections and broad emission bandwidth. Bi4Ge3O12 (eultine type BGO) - well known scintillator material, also a rare-earth host material, photorefractive waveguides produced in it only using light ions in the past. Recently: MeV N+ ions and swift O5+ and C5+ ions, too*. Bi12GeO20 (sillenite type BGO) - high photoconductivity and photorefractive sensitivity in the visible and NIR good candidate for real-time holography and optical phase conjugation, photorefractive waveguides produced in it only using light ions. No previous attempts of ion beam fabrication of waveguides in it

M-line spectroscopic, spectroscopic ellipsometric and microscopic measurements of optical waveguides fabricated by MeV-energy N+ ion irradiation for telecom applications

Irradiation with N+ ions of the 1.5 - 3.5 MeV energy range was applied to optical waveguide formation. Planar and channel waveguides have been fabricated in an Er-doped tungsten-tellurite glass, and in both types of bismuth germanate (BGO) crystals: Bi4Ge3O12 (eulytine) and Bi12GeO20 (sillenite). Multi-wavelength m-line spectroscopy and spectroscopic ellipsometry were used for the characterisation of the ion beam irradiated waveguides. Planar waveguides fabricated in the Er-doped tungsten-tellurite glass using irradiation with N+ ions at 3.5 MeV worked even at the 1550 nm telecommunication wavelength. 3.5 MeV N+ ion irradiated planar waveguides in eulytine-type BGO worked up to 1550 nm and those in sillenite-type BGO worked up to 1330 nm

Potassium currents in the heart: functional roles in repolarization, arrhythmia and therapeutics

L

Complex electrophysiological remodeling in postinfarction ischemic heart failure

K

Solvent Effects on the Steady-state Absorption and Fluorescence Spectra of Uracil, Thymine and 5-Fluorouracil

We report a comparison of the steady-state absorption and fluorescence spectra of three representative uracil derivatives (uracil, thymine and 5-fluorouracil) in alcoholic solutions. The present results are compared with those from our previous experimental and computational studies of the same compounds in water and acetonitrile. The effects of solvent polarity and hydrogen bonding on the spectra are discussed in the light of theoretical predictions. This comparative analysis provides a more complete picture of the solvent effects on the absorption and fluorescence properties of pyrimidine nucleobases, with special emphasis on the mechanism of the excited state deactivation

Absorption of UV radiation by DNA: Spatial and temporal features

International audienceThe present review focuses on studies carried out by our group on the interaction of UV radiation with DNA. In particular, we examine the way that the energy acquired by DNA helices following direct absorption of UVC radiation is extended spatially and how its effects evolve during the time. These effects depend on the base sequence and can be revealed by the study of model helices. The experimental results were obtained by optical spectroscopy, used in a refined way which allows detection of very weak absorbance changes (10−3) as well as of intrinsic emission from DNA components whose fluorescence quantum yields are as low as 10−4. Measurements were performed both under continuous irradiation and using pulsed excitation which permitted us to follow early events, occurring from 10−14 to 10−1 s. The experiments were guided by theoretical calculations. The spatial features concern the extent of the excited states formed immediately upon UV absorption; these were shown to be delocalized over several bases under the effect of electronic coupling. Moreover, thanks to the spectral fingerprints governed by the electronic coupling; we probed local denaturation induced on a double helix following formation of cyclobutane dimers. Regarding the temporal features, three different topics are presented: (i) ultrafast excitation energy transfer occurring among the bases in less than 100 fs, (ii) electron ejection from DNA upon absorption of one photon at 266 nm and (iii) formation of (6-4) photo-adducts involving a reaction intermediate. The most important message emerging from these studies is that DNA bases may adopt a collective behaviour versus UV radiation. Furthermore, time-resolved studies unravel processes which are undetectable by investigations using continuous irradiation. All these pieces of information change our understanding of how DNA damage occurs upon absorption of UV radiation

Optimizing Population Variability to Maximize Benefit.

Variability is inherent in any population, regardless whether the population comprises humans, plants, biological cells, or manufactured parts. Is the variability beneficial, detrimental, or inconsequential? This question is of fundamental importance in manufacturing, agriculture, and bioengineering. This question has no simple categorical answer because research shows that variability in a population can have both beneficial and detrimental effects. Here we ask whether there is a certain level of variability that can maximize benefit to the population as a whole. We answer this question by using a model composed of a population of individuals who independently make binary decisions; individuals vary in making a yes or no decision, and the aggregated effect of these decisions on the population is quantified by a benefit function (e.g. accuracy of the measurement using binary rulers, aggregate income of a town of farmers). Here we show that an optimal variance exists for maximizing the population benefit function; this optimal variance quantifies what is often called the "right mix" of individuals in a population